# Fluorescent Self-Supporting Composite Film Formed from Chitosan and the Neutral Poly(3-hexylthiophene-co-1,4-phenylene) Polymer with Enhanced Dispersion Properties for a Small Molecule

## TL;DR

A new composite film made from chitosan and a polymer shows improved ability to detect and adsorb a small molecule through fluorescence changes.

## Contribution

A novel composite film combining chitosan and a neutral conjugated polymer enhances small molecule dispersion and sensing.

## Key findings

- The composite film shows better dispersity of 4-AAB compared to pure chitosan films.
- Fluorescence quenching and emission changes in the film correlate with 4-AAB concentration.
- PTPh increases free volume and enables faster isomerization of 4-AAB in the composite film.

## Abstract

A composite film of chitosan (Ch) with a neutral conjugated
polymer,
poly(3-hexylthiophene-co-1,4-phenylene) (PTPh), was
developed to combine the adsorption capacity of Ch with the fluorescence
sensitivity of PTPh. Characterization of the films using thermogravimetric
analysis, microscopy, and infrared, absorption, and fluorescence spectroscopies
revealed that the dispersity of the target small molecule, 4-aminoazobenzene
(4-AAB), was improved in the composite film compared to the pristine
Ch film as evidenced in microscopy studies. In the presence of 4-AAB,
the Ch/PTPh film exhibited fluorescence quenching at low 4-AAB concentrations
and changes in emission spectra at higher concentrations. Photoisomerization
studies suggested that the improved dispersity of 4-AAB in the composite
film is due to an increase in the free volume provided by PTPh, with
faster cis-to-trans isomerization observed when PTPh was present.
Proof-of-concept adsorption experiments showed that the composite
film adsorbed 4-AAB from an aqueous solution, leading to a change
in the emission properties of the film. This qualitative characterization
uncovered a dual role for the conjugated polymer in the composite
film: the addition of the polymer changed the morphology and robustness
of the film, and the polymer also provides the fluorophore to sense
adsorbed molecules over a wide range of 4-AAB concentrations. These
results show that the strategy of incorporating water-insoluble polymers
at low concentrations into a versatile biopolymer leads to enhanced
functionalities of a composite material.